2 What is an fault arc - Die BG ETEM

Annex 6
Fig. A6.2: Current-time characteristics of NH fuses gG 400 V AC
A6.3.2 Work in house installations
A further example to be considered are live working activities in the house installations
behind the house service box fuses.
The installation is supplied by an house connection box with a rated current of
IN = 63 A at UrN = 400 V. From the short-circuit calculation a prospective short circuit
current of I"k3p = 4 kA is obtained. The supply is protected by a 63 A NH gG fuse.
First, the clearing time of the fuse will be determined. This has to be based on the
actual arcing fault current. The guide value of 0.5 is assumed for taking into account
the current attenuation 1 , meaning that the arc current is half of the size of the
prospective short circuit current, i.e. 2 kA. From the fuse characteristic (Fig. A6.2) a
release time tk lower than 10 ms can be read for a current of 2 kA. The fuse behaves
current-limiting, thus an exact clearing time cannot be predicted. For safety reasons
the short circuit duration is appointed to tk = 10 ms in such cases.
The normalized arc power will be determined also by means of the guide values
applicable for low-voltage systems (0.22 to 0.27): a value kP = 0.25 is chosen 2 .
1 As mentioned above, experience shows that the current attenuation in LV systems ranges between 0.5 and 1.0 in most of the
cases.
2 Experience shows in accordance with the typical conductor spacing in end-use LV equipment that guide values of normalized
arc power are in this range; kP = 0.25 characterizes very well most of these conditions.
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